Photo-charging of xerographic plates



Dec. 2, 1969 w, ROTH ETAL 3,481,669

PHOTOCHARGING OF XEROGRAPHIC PLATES Filed March 1, 1965 INVENTORS.WALTER ROTH CHARLES F. GALLO BY ALGIRD LEIGA a; II,

A TTOR/VEYS United States Patent 3,481,669 PHOTO-CHARGING OF XEROGRAPHICPLATES Walter Roth, Rochester, Charles F. Gallo, Fairport, and Algird G.Leiga, Pittsford, N.Y., assignors to Xerox Corporation, Rochester, N.Y.,a corporation of New York Filed Mar. 1, 1965, Ser. No. 436,145 Int. Cl.G03g 13/00 U.S. Cl. 355-3 12 Claims ABSTRACT OF THE DISCLOSURE Axerographic apparatus is disclosed wherein charging and exposure areaccomplished with the same source of electromagnetic radiation. Theradiation means comprises uniformly exposing means having a wavelengthwhich is not substantially longer than the threshold wavelength forelectron transmission from the photoconductive insulating layer of thexerographic plate and means producing a pattern of electromagneticradiation having a wavelength to which the photoconductive insulatinglayer shows photoconductive response.

This invention relates to xerography and, more specifically, to a methodand apparatus for charging and exposing a xerographic plate.

Xerographic ofiice copying has undergone an extremely large growth inthe past few years. In this copying technique, as originally disclosedby Carlson in U.S. Patent 2,297,691, and as further amplified by manyrelated patents in the field, a photoconductive insulating layer makingup a part of a xerographic plate is first given a uniform electrostaticcharge over its entire surface to sensitize it and is then exposed to alight image Which selectively drains away the charge in illuminatedareas of the photoconductive insulator, leaving behind charge in thenonilluminated areas to form a latent electrostatic image. This latentimage is then made visible by developing it through the deposition offinely divided, electroscopic, marking material on the surface of thephotoconductive insulating layer, as a result of which the markingmaterial conforms to the pattern of the latent image. The markingmaterial is generally made up of a powdered mixture of a thermoplasticand a colorant, such as a dye or. pigment, and is known in the art astoner. Where the photoconductive insulator is reusable, this visibletoner image is transferred to a second surface, such as a sheet ofpaper, after development and fixed in place on the paper to form apermanent visible reproduction of the original. Where, on the otherhand, a cheap, nonreusable photoconductive insulating material isemployed, the toner particles are fixerin place directly on its surfacewith the elimination of the transfer step from the process.

Although this process has been very successful commercially, certaindifficulties still exit with it. Consider, for example, the charging ofthe xerographic plate. A number of techniques have been developed forcharging and the technique which has gained widest commercial acceptanceis'corona charging, as more fully described in U.S. Patents 2,588,699 toCarlson and 2,777,957 to Walkup. Essencial electrostatic office copyingmachine, it suffers from certain inherent difficulties by the very factthat it operates on the principle of an ionizing electric fielddischarge. In certain instances, for example, the conditions of theatmosphere between the corona generating electrode and the xerographicplate to be sensitized can make important differences in theeffectiveness of plate sensitization. Thus, reduced air pressure, widechanges in relative humidity, large amounts of impurities in the air andother factors may have relatively important effects upon the level ofcharge which is deposited upon the plate with the charging voltage heldconstant. In addition, dust and other atmospheric impurities within themachine may deposit on the corona discharge electrodes, thereby limitingtheir effectiveness. Furthermore, high voltage power supplies withspecialized control circuits are often required in this type ofcharging. Thus, for example, in order to charge a plate surface to apotential of from about 600 to 800 volts, 2. potential of 4,000 to10,000 volts may be required on the corona discharge electrode. Sincecorona charging is the technique of choice in commercial devices, it caneasily be appreciated that other charging techniques known in the artsuffer from similar and even more troublesome difficulties.

Accordingly, it is an object of this invention to provide a novelxerographic charging method.

It is also an object of this invention to provide a novel xerographiccharging apparatus.

Yet another object of this invention is to provide a xerographiccharging technique employing the principle of photoemission.

A still further object of this invention is to provide a simplifiedxerographic apparatus in which plate charging and exposure areaccomplished with the same source of electromagnetic radiation.

These and still other objects may be accomplished in accordance with thepresent invention by exposing the xerographic plate to be charged to asource of light or other electromagnetic radiation whose Wavelength issufficiently short so that the energy of a photon of this radiationexceeds the work function of the xerographic plate to be charged. Theuse of light or other electromagnetic radiation of this thresholdwavelength or lower transfers suflicient energy to electrons in thephotoconductor to enable them to escape through the potential energybarrier at the surface of the photoconductor. The depletion of electronsfrom the surface of the photoconductor by photoelectric emission leavesthe plate with a net positive charge which continues to build up duringexposure as more electrons are emitted. In order to prevent the platefrom recapturing emitted electrons by virtue of its increasing positivecharge, a foraminous or transparent grid is provided adjacent thephotoconductive surface and a positive potential is applied to it withrespect to the photoconductor so as to capture emitted electrons. Thepotential applied to this electrode may be used to control the voltageto which the plate is charged. This voltage will not substantiallyexceed the electrode voltage because if it does, the plate will bepositive with respect to the electrode and will tend to recaptureemitted electrons.

The invention also contemplates the use of a relatively broad spectrumlight source which, with proper filtering, may be used for both uniformcharging and image-wise exposure of the photoconductor in thexerographic process, thereby eliminating one component of theconventional system. This is accomplished by filtering out all but theshort wavelength, high energy light from the source for charging theplate and then using the longer wavelength from the high UV and/orvisible portions of the spectrum for image-wise exposure. The shortwavelength light charges the photoconductor by photoelectric emission,but does not expose it significantly because the plate does not showmuch, if any, photoconductive response to this short wavelength oflight. On the other hand, the visible portion of the spectrum and longerUV will discharge the plate in image-wise configuration because platesshow high-er photoconductive response to these wavelengths of light but,at the same time, these wavelengths -do not have sufficient energy tocause photoelectric emission so as to charge the plate.

Photocharging is also employed in a preferred embodiment in conjunctionwith a plate having hole traps at least in the upper portion of the bulkof the photoconductor so as to improve its charge retention ability.This plate is exposed either with penetrating radiation or from itsirear surface through a transparent base so that the traps do notcapture the holes of the hole-electron pairs created on light exposureas this would prevent the formation of an electrostatic image byexposure.

The nature of the invention will be more easily understood when it isconsidered in conjunction with the accompanying drawings of an exemplarypreferred embodiment of the invention wherein:

FIG. 1 is a side sectional view of a simplified xerographic platecharging device and FIG. 2 is a side sectional view of a completexerographic copying apparatus employing the same light source for bothcharging and exposure of the image on the plate.

Referring now to FIG. 1, there is seen a xerographic plate in thisincluding a conductive base 11 and a photoconductive insulating layer12. It is to be noted that the electrically conductive base 11 is notnecessarily included in the system but may be employed to facilitatemaking electrical connection with the base of the photoconductiveinsulating layer 12. Immediately above the photoconductive insulatinglayer 12 of the plate is an electrode made up of a transparent quartzlayer 13 overcoated with an extremely thin, optically transparent,electrically conductive layer 14 of tin oxide or any other suitablematerial. The conductive portion of the electrode is con- .nected to thepositive side of a DC potential source 16 with the negative sideconnected to the conductive base 11 which may be grounded. Above theelectrode, there is positioned a light source 17 and a reflector 18positioned so as to reflect light through the electrode to thephotoconductive insulating layer. The light source 17 and reflector .18need not necessarily be positioned so that the light passes through theelectrode; however, since the electrode should be positioned in fairlyclose proximity to the surface of the photoconductive insulating layerso as to most efficiently capture emitted electrons therefrom when thelight strikes the plate, it is ditficult to position the light source insuch a way that the light will strike the plate directly without passingthrough the electrode. It is to be understood, however, that theinvention contemplates positioning the electrode in many alternativelocations even in back of or on the surface of the light source. Sinceit is a function of the electrode then, in most instances, to bothcapture emitted electrons and pass photons of the charging light, otheralternative structures may also be used in place of the electrodes shownon the drawings. A typical alternative structure of this type is a gridor screen of stainless steel, copper, brass or any other suitableconductive material. Although the charging light passes only through theopenings in the screen and not through the opaque or wire areasthemselves, in fact, this type of a screen grid constitutes a preferredform of electrode for use in connection with this invention because itproduces a very fine pattern of discontinuous charge over the surface ofthe plate in small discrete islands. This type of charge pattern isespecially valuable in developing large, solid, dark areas or continuoustone images. With ordinary charging in which the charge pattern isoriginally uniform over the whole plate even when it is examined on amicroscopic scale, low contrast originals do not produce high potentialgradients except at their extreme edges and, accordingly, the centers ofthese images tend to be hollow and not filled in unless complexspecialized developing methods are used. This discontinuous chargepattern, however, provides a great multiplicity of high potentialgradients over the whole surface of the plate so that even when large,solid, dark areas or continuous tone originals are used to expose theplate the latent electrostatic image produced will include small highpotential gradients throughout the image area, resulting in greatlyimproved development even when conventional developing techniques areemployed. The. screen used may include openings which are square, round,linear, irregular etc. in shape and may vary widely in size andfrequency. Typical structures include those having openings which coverfrom 20-80% of the screen area and vary in frequency from 20 to 500 perinch. Because of the macroscopic uniformity of the charge pattern evenwhen it is made through such a screen, all such patterns will bereferred to as uniform throughout this specification and the appendedclaims.

As explained above, the selection of the photoconductor to be employedand the light source to be employed are interrelated and each should beconsidered when selecting the other because the light source must becapable of supplying light which is approximately shorter than thethreshold wavelength, which may be defined as hxc/ work function of thephotoconductor; where h is Plancks constant and c is the speed of light.Taking the amorphous form of selenium as an example, the thresholdwavelength in angstrom units would have to be shorter than 12,395divided by the voltage equivalent of the work function for seleniumwhich is equal to a wavelength of about 2,620 angstrom units. It shouldbe noted that this is only an approximation and that slightly higherwavelengths may be employed because the work function for the materialsare calculated at absolute zero and the materials are ordinarilyoperated at higher temperatures, resulting in higher internal excitationof electrons Within the photoconductor. Although even photoconductorswith fairly high work functions may be employed in connection with theinvention and charged by using fairly short wavelength radiation, sayfor example in the 2,000 angstrom unit region, the use of the shorterwavelengths may require the exercise of carein the selection of theatmosphere between the charging light and the photoconductor'becausesome gases will absorb the short wavelengths preventing theirtransmission to the surface of the photoconductor.'Thus, for example, ithas been found that oxygen tends topartially absorb at about 2,000angstrom units or shorter wavelengths. Of course, this consideration maybe disregarded with the devices to be used in a vacuum or partial vacuumsuch as existsin outer space. When the system is used on earth it may beplaced in a container whose surface ex' tends almost to the surface ofthe photoconductor with either a vacuum or a non-absorbing gas in thecontainer or the area between the light source and the photoconductormay be continuously flushed with a non-absorbing gas such as nitrogen.

1 Any suitable photoconductive insulatinglayer may be used in carryingout the invention. Typical photoconductive insulating layers include:amorphousselenium, alloys of sulfur, arsenic or tellurium with amorphousselenium, amorphous selenium doped with hole trapping materials such asthallium, cadmium sulfide, cadmium selenide, etc., particulatephotoconductive materials such as zinc sulfide, zinc cadmium sulfide,French process zinc oxide, metal-free phthalocyanide, cadmium sulfide,cadmium selenide, zinc silicate, cadmium sulpho-selenide, linearquinacridones, etc., dispersed in an insulating inorganic film formingbinder such as a glass or an insulating organic film forming binder suchas an epoxy resin, a silicone resin, an alkd resin, a styrene-butadieneresin, a wax or the like. Other typical photoconductive insulatingmaterials include: blends, copolymers, terpolymers,

etc., of photoconductors and non-photoconductive materials which areeither copolymerizable or miscible together to form solid solutions andorganic photoconductive materials of this type include: anthracene,polyvinylanthracene, anthraquinone, oxidiazole derivatives such as2,5-bis-(p-arnino-phenyl-l), 1,3,4-oxidiazole; 2- phenylbenzoxazole; andcharge transfer complexes made by complexing resins such aspolyvinylcarbazole, phenolaldehydes, epoxies, phenoxies, polycarbonates,melamines, etc., with Lewis acids such as phthalic anhydride;2,4,7-trinitrofluorenone; metallic chlorides such as aluminum, zinc orferric chloride; 4,4-bis(dimethylamino) benzophenone; chloranil; picricacid; 1,3,5-trinitrobenzene; l-chloroanthraquinone; bromal;4-nitrobenzaldehyde; 4-nit-rophenol; acetic anhydride; maleic anhydride;boron trichloride; maleic acid; cinnamic acid; benzoic acid tartaricacid; malonic acid and mixtures thereof.

Since the technique of this invention produces positive charging andsome photoconductors operate more effectively when charged to onepolarity or the other, those which operate best with positive chargingwill usually, but not always, be used. It is to be noted, however, thatselenium in its amorphous form with or without doping and alloys of theamorphous form of selenium constitute a preferred material forphotoconductive insulating layer 12, because of their extremely highquality image-making capability and relatively high light response whenpositively charged. The photoconductor may also be coated on or alloyedwith any suitable material to reduce its surface potential energybarrier or work function. This type of coating may be eitherphotoconductive or not, so long as it is insulating enough in the darkto prevent destruction of the latent electrostatic image which is formedduring the process. In fact the coating may even be impervious to lightof the wavelength used for exposure providing the photoconductor iscoated on a transparent base and exposed through this base.

In a preferred form of the invention, photoemissive charging is employedwith a plate having hole traps at least in the upper layer of the bulkof its photoconductor. The high energy light in the UV range which isused for photoemissive charging penetrates very shallowly into the bulkof the photoconductive layer causing electron emission therefrom andleaving behind holes very near the sur face but still in the bulk of thephotoconductor. Since photoconductors which operate most efiicientlywith positive charging have a fairly high hole mobility, the holescreated by the charging light tend to drain away to some extent throughthe bulk of the photoconductor to the conductive substrate. Accordingly,by employing such a hole trapping layer at least near the top of thephotoconductor the holes created by photoemissive charging are trappednear the surface thereby allowing a higher level of charge to be builtup on the plate. Although selenium in its amorphous form doped with asmall amount of elemental thallium (for example .05 by weight) admirablyperforms the hole trapping function, any suitable hole trapping materialmay be employed. Typical hole trapping materials other than thalliumdoped selenium include cadmium sulfide and cadmium sulfoselenide. Thehole trapping material may be employed throughout the whole thickness ofthe photoconductor or only as an upper layer on the order of a fewmicrons thick since either structure will serve the function of trappingholes in the bulk near the upper surface of the photoconductor. Itshould be noted, however, that with either structure exposure from theupper surface of the plate with conventional light sources rich in lightfrom the blue end of the visible spectrum and the longer UV wavelengthsis to be avoided because light of these wavelengths is absorbed stronglyvery close to the upper surface in most photoconductors and,accordingly, can only form hole-electron pairs near that surface wherethe holes will be trapped thus preventing the formation of a latentimage. In the case of amorphous selenium doped with thallium in its toplayer or even throughout its bulk, this type of ab sorption will takeplace very near the surface in the UV- blue portion of theelectromagnetic spectrum. On the other hand, since selenium tends totransmit electromagnetic radiation from the orange-red end of thevisible spectrum and X-ray, penetrating actinic radiation of this typemay be used for exposure since the charge carriers are formed very closeto the conductive substrate. Consequently, even when the solid thalliumdoped selenium plate is used having short range hole mobility throughoutits bulk, most of the holes created by the exposing light source cantravel through the photoconductor to the conductive base for discharge,thereby allowing movement of the electron up toward the hole trappednear the surface of the plate to discharge it and form the desiredelectrostatic image.

Another technique for forming hole-electron pairs in the photoconductornear the plate substrate involves depositing the photoconductor on atransparent substrate such as tin oxide coated glass and then making theexposure through this transparent substrate. In this way light from theUV and blue portions of the spectrum may also be employed in forming theimage. In short, any exposure technique may be employed which generatesthe hole-electron pairs in the photoconductor close enough to thesubstrate so that the holes formed can move through the photoconductorto the substrate. The layered structure employing a thin layer of lessthan about 3 microns of a hole trapping material over a photoconductorwith a long range for holes is, of course, an even more preferred platestructure for use with photoemissive charging because of the holeelectron pairs may be created anywhere within the bulk of thephotoconductor below the trapping layer since this underlying layer hasa long enough range for holes so that they can reach the underlyingsubstrate even from the position just below the trapping layer. Anadditional and important advantage of using a plate with a trappinglayer is that photoemissive charging may be carried out using a broadspectrum light source including light from the blue end of the visiblespectrum and the longer UV as well as the shorter high energy UV withouta filter because, although the high energy UV causes electron emissionfrom the surface, the longer UV and blue light does not discharge theplate because of the hole traps near its surface.

An exemplary xerographic copying apparatus adapted to employ thephotocharging technique of this invention is shown in FIG. 2. Theapparatus consists of a xerographic drum generally identified as 19consisting of a grounded conductive substrate 21 and a photoconductiveinsulating layer of selenium 22 mounted thereon with the whole drumjournaled for rotation on a cylindrical shaft 23. The drum, when inoperation, is generally rotated at a uniform velocity in the directionindicated by the arrow in FIG. 2 so that portions of the drum peripheryfirst move past the charging unit which includes an opticallytransparent electrode 24 of the type described above in connection withFIG. 1 connected to a source of posiive potential 26. Above theelectrode is a wide spectrum light source 27 which puts out light in therelatively short wavelength portion of the UV, the longer UV and theblue end of the visible spectrum. Typical light sources of this typeoperate by continuous or pulsed electric discharge in atmospheres ofmercury, iodine, rare gases or metallic vapors. The light source 27 isincluded in a light-tight cabinet 28 with a slit 29 in one side of thecabinet. Also in-. cluded in the cabinet between the light source 27 andelectrode 24 is a filter 31 which passes the shorter wave-, length UVonly, thus filtering out the longer UV and visible portion of thespectrum. Electrode 24 and filter 31 may be combined by applying anoptically transparent conductive layer to the surface of the filter. Inthis instance a photoconductor such as amorphous selenium is employedhaving a work function such that the shorter wavelength UV causesphotoelectric emission of electrons therefrom and which also showsphotoconductive response to the longer UV and the blue end of thevisible spectrum. When the light source 27 is turned on the shortwavelength UV light of less than about 2,650 angstrom units passesthrough filter 31 and grid 24 to cause electron emission from theunderlying photoconductive layer 22, and grid 24 captures emittedelectrons by virtue of the electrical field applied to it from thepotential source 26 until the potential on the photoconductor approachesthat of the applied potential 26. At the same time, light from source 27in an unfiltered form also impinges on the surface of the original image32 to be reproduced. This original 32 is held on a cylindrical copy drum33 by grippers 34 and the drum is rotated in the direction shown by thearrow at the same peripheral speed as that imparted to the xerographicdrum. The trailing edge of the original 32 is held against the rotatingcopy drum 33 by springlike fingers 36. A similar and much more detailedshowing of a copy drum and associated mechanism for holding it andmoving it past a light source, which may be employed in connection withthe present invention, is shown in U.S. Patent 3,009,943 to Eichorn. Thelight from the unfiltered source 27 thus exposes the original image 32to be reproduced and is reflected out through slit 29, through lens 37,oif mirror 38, and then exposes the photoconductive insulating surface22 of the xerographic drum 19. The lens prevents the passage of shortwavelength UV and allows the passage of longer UV and visible lightbecause it is made of glass. Since the photoconductive insulatingselenium shows photoconductive response to visible and long UV, chargeis drained olf the plate surface in exposed areas to form a latentelectrostatic image thereon corresponding to the dark areas on theoriginal. Subsequent to charging and exposure, sections of thexerographic drum surface move past the developing unit, generallydesignated 41. This developing unit is of the eascade type whichincludes an outer container or cover 42 with a trough at its bottomcontaining a supply of developing material 43. The developing materialis picked up from the bottom of the container and dumped or cascadedover the drum surface by a number of buckets 44 on an endless drivenconveyor belt 46. This development technique, which is more fullydescribed in US. Patent 2,618,552 to Wise and 2,618,551 to Walkup,utilizes a two-element development mixture including a finely divided,colored, marking particles or toner and larger carrier beads. Thecarrier beads serve both to deagglomerate the fine tone particles foreasier feeding and charge them by virtue of the relative positions ofthe toner and carrier material in the triboelectric series. The carrierbeads with toner particles clinging to them are cascaded over the drumsurface. The eletcrostatic field from the charge pattern on the drumpulls toner particles off the carrier beds serving to develop the image.Then the carrier beads, along with any toner particles not used todevelop the image, fall back into the bottom of the container 42 and thedeveloped image moves around until it comes into contact with a copy web47 which is pressed up against the drum surface by two idle rollers 48so that the web moves at the same speed as the periphery of the drum.Toner in the developing mixture is periodically replenished from a tonerdispenser not shown. A transfer unit 49 is placed behind the web andspaced slightly from it between rollers 48. This charging unit 49 isconnected to a source of high positive DC. potential identified as 51and includes a corona discharge wire 52 surrounded by a conductive metalshield 53. The voltage is selected to be of such a value that it willcause a corona discharge onto the back surface of the copy web 47, andthis charge is of the same polarity as the charge initially deposited onthe drum and opposite in polarity to the charge on the toner particlesutilized in developing the image. The discharge deposited on the back ofweb 47 pulls the toner particles away from the drum by overcoming theforce of attraction between the particles and the charge on the drum.

Many other transfer techniques known in the art can be utilized inconjunction with this invention. For example, a roller connected to ahigh potential source opposite in polarity to the toner particles may beplaced immediately behind the copy web or the web itself may be adhesiveto the toner particles. After transfer of the toner image to web 47 theweb moves beneath a fixing unit 54 which serves to fuse or permanentlyfix the toner image to the web. In this case a resistance heating typefixer is illustrated; however, here again other techniques known in theart may be utilized for fixing including the subjection of the tonerimage to a solvent vapor, spraying of the toner image with an adhesiveovercoating, subjection of the toner image to electromagnetic radiation,etc. After fixing the web is rewound on a coil 56 for later use. Oncethe drum has passed the transfer station it continues around and movesbeneath a cleaning brush 57 which prepares it for a new cycle ofoperation.

What is claimed is:

1. An apparatus for forming a latent electrostatic image comprising axerographic plate including a photoconductive insulating layer, means touniformly expose said xerographic plate to electromagnetic radiationhaving a wavelength which is not substantially longer than the thresholdwavelength for electron emission from said photoconductive insulatinglayer, said source being positioned adjacent to said photoconductiveinsulating layer, means to collect electrons emitted from saidphotoconductive insulating layer and means to expose said xerographicplate to an electromagnetic radiation pattern having a wavelength towhich said photoconductive insulating layer shows photoconductiveresponse.

2. An apparatus for uniformly charging the photoconductive insulatinglayer of a xerographic plate comprising a source of electromagneticradiation having a wavelength which is not substantially longer than thethreshold wavelength for electron emission from said photoconductiveinsulating layer, said source being positioned adjacent to saidphotoconductive insulating layer, means to collect electrons emittedfrom said photoconductive insulating layer, and an optical filterbetween said photoconductive insulating layer and said source ofelectromagnetic radiation, said filter being adapted to block thepassage of radiation of a wavelength to which the photoconductiveinsulating layer shows photoconductive response.

3. An apparatus according to claim 2 in which said collecting meanscomprises an electricallyconductive electrode opposite saidphotoconductive insulating layer and a positive polarity potentialsource connected to said electrode.

4. An apparatus according to claim 2 in which said collecting meanscomprises a foraminous electrically conductive electrode connected to apositive polarity electrical potential source.

5. Apparatus for forming a latent electrostatic image comprising axerographic plate including a photoconductive insulating layer, a broadband source of electromagnetic radiation adjacent said photoconductiveinsulating layer which emits in a wavelength range which includes awavelength substantially shorter than the threshold wavelength forelectron emission from said photoconductive insulating layer and alonger wavelength to which said photoconductive insulating layer showsphotoconductive response, means to move said xerographic plate throughan imaging path, first filter means to block radiation of wavelength towhich said photoconductive insulating layer shows photoconductiveresponse, said first filter means being positioned between saidelectromagnetic radiation source and a first portion of said imagingpath, means to move an original image to be reproduced past a pointadjacent said electromagnetic radiation path and to project the lightreflected from the original at that point through an optical pathincluding a second filter and onto the photoconductive insulating layerin a second portion of said imaging path, said second filter means beingof a type which blocks electromagnetic radiation of a Wavelength nolonger than the threshold wavelength for said photoconductive insulatinglayer.

6. Apparatus for forming a uniform charge for xerographic reproductioncomprising xerographic plate including a photoconductive insulatinglayer at least the surface layer of which has a short range for holes, asource of electromagnetic radiation a wavelength which is notsubstantially longer than the threshold wavelength for electron emissionfrom said xerographic plate, said source being positioned to irradiatesaid xerographic plate and a positively biased collecting electrodeopposite said photoconductive insulating layer adapted to collectelectrons emitted therefrom.

7. Apparatus according to claim 6 in which the photoconductiveinsulating layer of said xerographic plate is deposited on a transparentsubstrate and further including means to expose said xerographic plateafter charging to an image to be reproduced through said transparentsubstrate with actinic electromagnetic radiation.

8. An apparatus according to claim 6 further including means to exposesaid xerographic plate after charging, said exposure means including asource of penetrating actinic electromagnetic radiation whereby holeelectron pairs will be formed in said plate upon exposure at asufficient depth so that said holes can reach the conductive substrateof said plate.

9. A method for uniformly charging a xerographic plate including aphotoconductive insulating layer at least the surface layer of which hasa short range for holes comprising exposing said surface layer to asource of electromagnetic radiation, said source including radiation ofa wavelength which is not substantially longer than the thresholdwavelength for electron emission from said xerographic plate andapplying an electrical field adjacent said plate, said field being of apolarity to move emitted electrons away from said plate whereby it isleft with a net positive charge.

10. A method of forming a latent electrostatic image comprisinguniformly exposing a xerographic plate including a photoconductiveinsulating material on a conductive substrate, at least the surfacelayer of which has a short range for holes, with a source ofelectromagnetic radiation, said source including radiation of awavelength which is not substantially longer than the thresholdwavelength for electron emission from said xerographic plate,

applying an electrical field adjacent said xerographic plate, said fieldbeing of a polarity to move emitted electrons away from the free surfaceof said xerographic plate so that it is left with a uniform net positivecharge, and exposing said charged xerographic plate to an image to bereproduced with penetrating actinic electromagnetic radiation so as togenerate hole-electron pairs adjacent the surface of saidphotoconductive insulating layer most remote from said surface layer.

11. A method of forming a latent electrostatic image comprisinguniformly exposing a xerographic plate including a photoconductiveinsulating material on a transparent conductive substrate, at least thesurface layer of which has a short range for holes, with a source ofelectromagnetic radiation, said source including radiation of awavelength which is not substantially longer than the thresholdwavelength for electron emission from said xerographic plate, applyingan electrical field adjacent said xerographic plate, said field being ofa polarity to move emitted electrons away from the free surface of saidxerographic plate so that it is left with a uniform net positive charge,and exposing said charged xerographic plate through its conductivesubstrate.

12. A method of forming a latent electrostatic image comprisinguniformly exposing a xerographic plate including a photoconductiveinsulating material on a conductive substrate to electromagneticradiation with an energy level in excess of the work function of saidphotoconductive insulating material, applying an electric field adjacentsaid xerographic plate, said field being of a polarity to move emittedelectrons away from the free surface of said pho toconductive insulatingmaterial whereby it is left with a uniform net positive charge, andexposing said uniformly charged xerographic plate to an image to bereproduced of electromagnetic radiation to which said photoconductiveinsulating layer shows photoconductive response.

References Cited UNITED STATES PATENTS 2,990,280 6/1961 Giaimo 9613,057,997 10/ 1962 Kaprelian 1.7 3,254,215 5/1966 Cliphant 25049.53,254,998 6/ 1966 Schwertz 961 3,322,539 5/1967 Redington 961.l

JOHN M. HORAN, Primary Examiner

